This invention relates generally to carbon dioxide (CO2) sensors.
Electrochemical CO2 sensor using a Severinghaus electrode are known. Although widely accepted for measurements of dissolved CO2, the Severinghaus type sensor is not suitable for gas phase measurements. Sensor drift arising from loss of electrolyte by evaporation or failure due to puncture of the sensor's Teflon membrane are among the most serious factors limiting the use of this type of sensor. A number of sensors are commercially available for gas phase measurements of CO2 based on non-dispersive infrared (IR) absorption. Gaseous CO2 exhibits a characteristic absorption band in the mid-IR that can be used to determine gas phase concentrations according to Beer's law. Although sensitive, the IR devices require expensive detectors and light sources, and must include sample cell heaters or water vapor filters under conditions where water condensation can occur in order to avoid interference in their readings.
CO2 sensors that provide a detectable indication of the presence of an elevated proportion of carbon dioxide in gaseous state, where the sensor has a substrate coated by an intimate mixture of a transparent plasticised polymer vehicle, and an indicator material which undergoes a color change on exposure to carbon dioxide, the mixture disposed over a substrate, have been disclosed.
There is a need for CO2 sensors for carbon dioxide in gaseous state, where the sensor does not need a transparent polymer vehicle or a plasticizer and the sensor is easily read.
Two exemplary embodiments of applications where there is a need for improved CO2 sensors are described below.
The role of arterial carbon dioxide on vasodilation makes it a critical parameter in controlling tissue perfusion and oxygen delivery particularly during prehospital care of patients requiring mechanical ventilation. Proper ventilation leading to early correction and/or maintenance at normocapnia levels in patients with severe traumatic brain injury has been shown to significantly reduce mortality rates in these high risk subjects. These results support the development of improved ambulatory mechanical ventilation technologies including the development of more accurate noninvasive means of estimating PaCO2.
Presently, the existing methods for noninvasive estimation of PaCO2 include measurements of end-tidal carbon dioxide (PETCO2) or transcutaneous CO2 sensing electrodes. Issues with correlation and/or accuracy in comparative studies against invasive blood gas analysis have raised questions about the utility of these devices, particularly when applied to adult subjects. Discrepancies between PETCO2 readings and PaCO2 measurements by blood gas analyzers are primarily attributed to the presence of pulmonary dead space volume and physiological conditions that can exacerbate dead space volume including obstructive pulmonary pathology, hypovolemia, atelectasis and mechanical ventilation. Studies of Severinghaus-type transcutaneous CO2 electrodes have shown better correlation with blood gas analysis (BGA) values, but there remain problems with inaccuracies arising from calibration drift due to evaporative loss of electrolyte and slow response times which introduce a time lag in readings taken with this type of sensor. The use of localized heating has long been used to improve the response time of transcutaneous sensors by increasing arterilization and epidermal permeability in the area under investigation. However, local heating does not appreciably reduce the two to three minute response times of CO2 electrodes which introduces a time lag in readings that can be misinterpreted as an error when compared with BGA readings especially if blood samples occurs during a period of rapid change in the arterial CO-level. A CO2 gas sensor that can be reliably used for noninvasive monitoring of arterial CO2 does not exist and new ways to approach the problem are needed.
In another exemplary embodiment, CO2 sensors find applications in rebreathers used by divers. Divers use a closed circuit Underwater Breathing Apparatus (UBA), also known as rebreather, for many of their deep diving operations and for training. Although there are several design variations of the diving rebreather, all types have a gas-tight loop that the diver inhales from and exhales into. The diver breathes through a mouthpiece that is connected to one or more tubes bringing inhaled gas and exhales gas to a breathing bag. The loop also includes a scrubber containing carbon dioxide absorbent to remove from the loop the carbon dioxide (CO2). The exhaled gases are forced through the chemical scrubber which removes the carbon dioxide from the gas mixture and leaves the oxygen and other gases available for re-breathing. Scrubber failure, which can result from many causes, leads to black-out and hence is very dangerous to the diver. It would be very useful to monitor the CO2 in the rebreather so that the scrubber can be replaced before the CO2 levels get dangerously high. Currently, no such CO2 sensor exists for the use of deep sea divers.
There is a need for CO2 sensors for carbon dioxide in gaseous state, where the sensor does not need a transparent polymer vehicle or a plasticizer and the sensor is easily read, has a fast response time can be reliably used for noninvasive monitoring.